WOOD STOCK IN NEOTROPICAL STREAMS: QUANTIFYING AND COMPARING INSTREAM WOOD AMONG BIOMES AND REGIONS
In-stream wood plays important chemical, physical and ecological functions in aquatic systems, benefiting the biota directly and indirectly. However, human activities along river corridors have disrupted wood recruitment and retention leading to reduced in-stream wood amounts. In the tropics, where wood is assumed to be more transient, the expansion of agriculture and infrastructure might be reducing in-stream wood stock even more than in the better studied temperate streams. However, research is needed to augment the small amount of information about wood in different biomes and ecosystems of neotropical streams. Here we present the first extensive in-stream wood assessment in wet-tropical Amazon and semi-humid-tropical Cerrado (the Brazilian savanna) catchments, describing the in-stream wood loads and size distributions. We also compare neotropical wood stocks with those from temperate streams, first contrasting with literature data and then with a comparable dataset from USA temperate biomes. Contrary to our expectations Amazon and Cerrado streams had similar wood loads, which are lower than the world literature average, but similar to those found in comparable temperate forest and savanna streams in USA. Our results indicate that the field survey methods and the wood metric examined are highly important when comparing different datasets. But when properly compared, we found that most of the wood in temperate streams is made-up of a small number of large pieces, whereas wood in neotropical streams is made up of larger number of small pieces that produce similar total volumes. The character of wood volumes among biomes is linked more to the delivery, transport and decomposition mechanisms than to the total number of pieces. Future studies should further investigate the potential in-stream wood drivers in neotropical catchments in order to better understand the differences and similarities here detected between biomes and bioclimatic regions.